Problem: Solve for $z$ : $12 = z + 20$
Subtract $20$ from both sides: $ \begin{eqnarray} \\ \hphantom{12 {- 20}} &\hphantom{=}& \hphantom{z} \\ 12 &=& z + 20 \\ \\ {-20} && {-20} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ 12 {- 20} &=& z \\ \end{eqnarray} $ Simplify. $ z = -8$